1. Field of the Invention
The present invention concerns a photosensitive semiconductor element, a photosensitive semiconductor element arrangement, a photosensitive circuit and a method of regulating flames using the photosensitive semiconductor element or the photosensitive semiconductor element arrangement.
2. Description of the Prior Art
Photosensitive semiconductor elements, which are also known in the form of a photodiode or phototransistor, already exist as so-called pn-junctions of suitably doped semiconductor devices and are described for example in "Physics of Semiconductor Devices", 2nd edition, S. M. Sze, 1981, published by John Wiley & Sons, Inc., Part V, Chapter 13, "Photodetectors". Those photodetectors or photosensitive semiconductor elements serve for detecting light, that is to say visible or invisible light such as for example UV-light or IR-light. The three most important parameters for describing the quality of such photosensitive semiconductor elements are sensitivity, that is to say spectral sensitivity (sensitivity in relation to given wavelengths), response time, that is to say the time between incidence of the light and detection of a sensor signal, and finally quantum efficiency.
Photosensitive semiconductor elements of that kind are already known. For example A. R. Jones: "Flame failure detection and modern boilers", J. Phys. E.: Sci. Instrum. 21 (1988), 921-928, describes what are referred to as ultravioletsensitive gas discharge tubes which have a very high level of sensitivity and which are intrinsically selective. Nonetheless that type of photodetector is very expensive and fragile, it is of large size and it requires a very high level of power supply.
Furthermore conventional silicon photodiodes are known, which are provided with external or integrated filters in order thereby to acquire a certain degree of selectivity. Those photodiodes are very small and almost unbreakable. Interference filters can be used to achieve a high level of selectivity in the UV-range, that is to say in order to filter out visible light and IR-light adjoining the visible range. Nonetheless such UV-photodiodes are highly expensive as the interference filters push up the costs and as a result make the overall circuitry an unattractive proposition in terms of use in products involving large production numbers. On the other hand however there are also interference filters in a low price category, but they exhibit poorer filter properties for visible light and IR-light adjoining the visible range, so that in this case the level of selectivity is inadequate for most uses.
A commercially available UV-photodiode with a high level of selectivity is described for example in R. S. Popovic et al "A silicon ultraviolet detector", Sensors and Actuators, A 21-A 23, 1990, 553-558, and is known from EP 0 296 371, wherein those UV-photodiodes are subjected to a very expensive and tedious optimization procedure in regard to doping of the corresponding semiconductor layers in order to achieve that high level of selectivity. As a result those photodiodes are highly expensive and are scarcely suitable for use in products involving large numbers.
Those previously known semiconductor elements for detecting light, that is to say for example photodiodes, have a substrate of silicon, in which an intermediate layer and an outer layer are at least partially embedded. The intermediate layer and the outer layer are distinguished by a different form of doping. They are therefore for example p- or n-doped in order to produce the pn-junction required for the photocurrent. To generate a light-dependent signal, that is to say for example the photocurrent, the intermediate layer and the outer layer form a photosensitive region in which the incident photons cause the formation of charge carriers, that is to say electrons and holes, which are in the conduction band and the valence band respectively of the semiconductor. Transportation of those charge carriers and possibly further generation of charge carriers (avalanche effect) gives rise to a photocurrent which can be measured by suitable contacts at the surface of those layers. Depending on the respective structure and doping of the corresponding semiconductor layers, generation, transportation and speed of the charge carriers can be influenced. In most cases however the qualitative demands are in conflict with the structural options for the production of such components.
Accordingly the conventional photosensitive semiconductor elements either afford a high level of selectivity, but in that case are very large and very expensive, or they are small and cheap but involve a low level of selectivity or lack of effectiveness so that they cannot be reliably used for appropriately relevant electronic circuits. For example certain photosensitive semiconductor elements are provided for monitoring circuits which are intended to guarantee a fail-safe performance. Hitherto no usable configurations which are at the same time cheap and simple to produce have been available.
For example, conventional photosensitive semiconductor elements for use for monitoring flames in burners of heating installations for private houses or industrial buildings, for fire alarm systems or for building monitoring systems at reasonable item prices with a sufficiently high level of selectivity and adequate quantitative effectiveness are not available at the present time.